The present invention relates to rotary regenerative heat exchangers generally used as air preheaters and more specifically to an improved rotor construction which combines the advantages of both modular and non-modular construction methods.
A rotary regenerative heat exchanger is employed to transfer heat from one hot gas stream, such as a hot flue gas stream, to another cold gas stream, such as combustion air. The rotor contains a mass of heat absorbent material which first rotates through a passageway for the hot gas stream where heat is absorbed by the heat absorbent material. As the rotor continues to turn, the heated absorbent material enters the passageway for the cold gas stream where the heat is transferred from the absorbent material to the cold gas stream.
In a typical rotary heat exchanger, such a rotary regenerative air preheater, the cylindrical rotor is disposed on a vertical central rotor post and divided into a plurality of sector-shaped compartments by a plurality of radial partitions, referred to as diaphragms, extending from the rotor post to the outer peripheral shell of the rotor. These sector-shaped compartments are loaded with modular heat exchange baskets which contain the mass of heat absorbent material commonly formed of stacked plate-like elements.
The rotors of such heat exchangers are either formed as non-modular, shop assembled rotors or as modular rotors. The non-modular rotors comprise a series of diaphragm plates each attached to the rotor post and extending out to the rotor shell thereby dividing the rotor into sectors. Further, each sector is divided into a number of compartments by stay plates extending between the diaphragms at spaced intervals. The modular heat exchange baskets are then loaded axially into these compartments from the top end (duct end). The non-modular rotors are labor intensive because the majority of the rotor structure is first shop-assembled and then at least partially disassembled for shipment. The result is more total time to manufacture and field install.
Modular rotors are composed of a series of shop-assembled sector modules which are then field-assembled into a complete rotor. Each sector module has a diaphragm plate on each side with these two diaphragms being joined by stay plates. When these modules are assembled into a rotor in the field, the diaphragm plates of adjacent modules are joined together to form a double plated diaphragm. Although the modular rotors require less time to field-install than non-modular rotors, they require twice as many individual diaphragm plates which take up gas flow area and allow less heat transfer area for the same size rotor and post diameter. Also, they are component intensive because of all the parts necessary to pin the adjacent modules to each other at diaphragm locations.
Most modular and non-modular rotor designs contain stay plates as previously described. The stay plates reinforce the rotor structure and support the baskets. Because the baskets are inserted axially and must fit in the stay plate compartments, the baskets must be undersized for easy installation and removal. Undersizing involves providing a gap around the perimeter of each basket. This reduces the free area of the basket available for heat transfer flow and creates flow bypass gaps around the baskets. The result is decreased air preheater efficiency and the selection of larger air preheaters for any particular performance requirements.